The invention relates to an electrochromic device and to its production.
Electrochromic devices comprising an electrochromic system are already known.
The electrochromic system of such devices commonly includes pairs of redox substancesxe2x80x94redox couplesxe2x80x94dissolved in an inert solvent. Conductive salts, light stabilizers and substances which influence the viscosity may additionally be present.
The redox couple used comprises one reducible and one oxidizable substance each. Both are colourless or have only a weak colouration. Under the influence of an electrical voltage, one substance is reduced and the other oxidized, with at least one becoming coloured in the process. After the voltage is switched off, the two original redox substances are formed once more, which is accompanied by the disappearance or fading of the colour. 
U.S. Pat. No. 4,902,108 discloses that suitable such redox couples are those where the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance, correspondingly, has at least two chemically reversible oxidation waves.
Electrochromic devices can find multivarious applications. For example, they may take the form of a rearview car mirror which when travelling at night can be darkened by applying a voltage, thus preventing the driver being dazzled by headlights of other vehicles (cf. e.g. U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A 0 435 689). Devices of this type may also be employed in window panes or car sun roofs where, following application of a voltage, they provide shade from the sunlight. Finally, it is possible to use such devices to construct a display device for the graphic representation of information in the form of letters, numbers and symbols, for instance.
Electrochromic devices normally consist of a pair of glass or plastic plates, one being mirrored in the case of a car mirror. One side of these plates is coated with a transparent, electroconductive layer, e.g. indium tin oxide (ITO). These plates are then used to construct a cell: to this end their facing, electroconductively coated side is attached, preferably by means of adhesive bonding, to an annular or rectangular sealing ring. The sealing ring establishes a uniform distance between the plates of, for example, from 0.01 to 0.5 mm. This cell is then filled via an aperture with an electrochromic solution and the cell is tightly sealed. By way of the ITO layer it is possible to contact the two plates separately.
The electrochromic systems known from the prior art comprise redox couples which following the reduction and oxidation, respectively, form coloured free radicals, cationic free radicals or anionic free radicals that are chemically reactive. As known, for example, from Topics in Current Chemistry, Vol. 92, pp. 1-44 (1980) such (ionic) free radicals may be sensitive to electrophiles or nucleophiles or else to free radicals. In order, therefore, to achieve a high level of stability in an electrochromic device comprising an electrochromic system of this kindxe2x80x94a system which is intended to withstand several thousand switching cyclesxe2x80x94it is necessary to ensure that the solvent used is absolutely free from electrophiles, e.g. protons, nucleophiles and oxygen. It must also be ensured that such reactive species are not formed by electrochemical processes taking place at the electrodes during operation of the electrochromic device.
Suitable solvents are known from the patents cited above.
In addition, however, the sealing and closure material of the electrochromic cell must be such that it does not comprise, give off or form in contact with the electrochromic system any reactive constituents which react with the electrochromic substances or with the species formed electrochemically from them, such as the abovementioned free-radical or ionic free-radical species or with the double-reduced or double-oxidized species which are always present in equilibrium or are formed electrochemically.
The back-reaction to RED1 and OX2 that is formulated in the above equation also takes place continuously away from the electrodes within the volume of the solution while the electrochromic device is in operation. Owing to the above-described hazards of degradation reactions of the (ionic) free radicals by electrophiles, nucleophiles or free radicals it is important, for the long-term stability of the display, that the back-reaction in accordance with the above equation is able to take place as rapidly as possible and without side reactions.
It has now been found that by coupling RED1 and OX2 via a covalent chemical bond, electron transfer is facilitated and hence the back-reaction in accordance with the above equation can be accelerated and side reactions avoided.
Other findings have been suitable adhesives, which can be used as a seal and as a closure of the electrochromic device after it has been filled, and appropriate curing conditions for these adhesives, so that none of the hazardous side reactions described above occur.
The present invention accordingly provides an electrochromic device comprising a pair of transparently and conductively coated glass or plastic plates, of which one may be mirrored and of which the conductive layer of one of the two plates or else of both can be subdivided into separate, individually electrically contacted segments which are joined together on the sides of their conductive coating by an adhesive bead into which spacers may be embedded, and where the volume formed from the two plates and the adhesive bead is filled with an electrochromic fluid and where the filling aperture or apertures required for introduction of the electrochromic fluid is or are sealed with an adhesive after filling has taken place, characterized in that a thermally or photochemically curing epoxy adhesive, or an epoxy adhesive which cures thermally after photochemical initiation, is used for the adhesive bead for constructing the electrochromic device and in that a photochemically curing acrylate adhesive or an epoxy adhesive which cures photochemically or following photochemical initiation at room temperature is used for sealing.